There is a growing trend across common law jurisdictions towards more structured sentencing processes. Many have implemented some form of sentencing guidelines in recent years; others are designing or considering introducing them. This article focuses on Ireland, a jurisdiction that has recently legislated for sentencing guidelines but does not have sufficient sentencing data available to inform key decisions on guideline design and implementation. It highlights the importance of sentencing data for guideline development and outlines how the lack of data on current sentencing trends and outcomes means that there is no frame of reference for current practice against which to design guidelines, nor assess their likely impact or gauge their effectiveness once introduced. The article examines the types of data that are needed to develop and monitor sentencing guidelines and sets out how they might be collected and analysed in Ireland. The Irish example serves as a case study for other jurisdictions with respect to the importance of sentencing data and the need to ensure that such data are available prior to the introduction of sentencing guidelines